1. Field of the Invention
This invention generally relates to a polymeric coating for an implantable device such as a drug eluting or delivery stent.
2. Description of the Background
Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient. One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer. One such polymeric carrier is a coating composition based on polylactic acid (PLA).
Many of the biocompatible polymeric carriers used in the art of drug delivery have relatively high glass transition temperatures (Tg) and relatively small range of elongation. For example, PLA of a molecular weight of 100,000 Daltons has a Tg of about 51° C., and a range of elongation of about 4%, which renders a coating formed of PLA hard and inflexible. When a composition of such a polymer is used for coating an implantable device, certain undesirable side effects may develop in the course of using such an implantable device because, in the process of implantation, an implantable device is often physically manipulated, e.g., expanded or deformed, leading to damages to the coating. FIGS. 1A and 1B show a stent coated with PDLLA after wet expansion that having some polymer peeling off at the high strain joints.
Therefore, there is a need for a coating of an implantable device that provides a controlled release of a bioactive agent and improved mechanical properties.
The compositions and the coatings formed thereof disclosed herein address the above described problems and needs.